1. Field of the Invention
The present invention relates to a method of making a material alloy for a rare-earth magnet, a method of making a material alloy powder for a rare-earth magnet, and a method for producing a sintered magnet using the rare-earth magnet material alloy powder.
2. Description of the Related Art
A neodymium-iron-boron based magnet has a higher maximum energy product than any of various types of magnets, and is relatively inexpensive. That is why such a magnet has been used more and more often as an important part of an HDD, an MRI or a motor in a broad variety of electronic devices.
A neodymium-iron-boron based magnet is a magnet including Nd2Fe14B type crystals as its main phase and is sometimes called an “R-T-B based magnet” more generically, where R is a rare-earth element, T is a transition metal element, most of which is Fe but which may also include Ni and Co, and B is boron. However, since B may be partially replaced with an element such as C, N, Al, Si and/or P, at least one element selected from the group consisting of B, C, N, Al, Si and P will be referred to herein as “Q” and a rare-earth magnet, which is usually called a “neodymium-iron-boron based magnet” will be referred to herein as an “R-T-Q based rare-earth magnet” more broadly. In an R-T-Q based rare-earth magnet, R2T14Q crystal grains form its main phase.
A material alloy powder for an R-T-Q based rare-earth magnet is often made by a process including a first pulverization process step for coarsely pulverizing the material alloy and a second pulverization process step for finely pulverizing the material alloy. For example, in the first pulverization process step, the material alloy is coarsely pulverized to a size of several hundreds of micrometers or less by a hydrogen decrepitation process. In the second pulverization process step, the coarsely pulverized material alloy (coarsely pulverized powder) is finely pulverized to a mean particle size of about several micrometers using a jet mill pulverizer, for example.
The magnet material alloy itself may be made by any of a number of methods, which are roughly classified into the following two types. The first type is an ingot casting process, in which a molten alloy with a predetermined composition is poured into a die and cooled relatively slowly. The second type is a rapid cooling process such as a strip casting process and a centrifugal casting process, in which a molten material alloy with a predetermined composition is rapidly cooled through a contact with a single roller, twin rollers, a rotary disk or a rotary cylindrical die, thereby making a solidified alloy, which is thinner than an ingot cast alloy, from the molten alloy.
In the rapid cooling process, the molten alloy is cooled at a rate of 101° C./s to 104° C./s. The rapidly cooled alloy made by the rapid cooling process has a thickness of 0.03 mm to 10 mm. The molten alloy starts to be solidified on the surface that has contacted with the chill roller (i.e., a roller contact surface). From the roller contact surface, crystal grows in the thickness direction into the shape of needles. The resultant rapidly cooled alloy has a microcrystalline structure including an R2T14Q crystalline phase having minor-axis sizes of 3 μm to 10 μm and major-axis sizes of 10 μm to 300 μm and R-rich phases dispersed on the grain boundary of the R2T14Q crystalline phase (i.e., a phase including a rare-earth element R at a relatively high concentration). The R-rich phases are nonmagnetic phases in which the concentration of the rare-earth element R is relatively high, and has a thickness (which corresponds to the width of the grain boundary) of 10 μm or less.
As the rapidly cooled alloy has been cooled in a shorter time than an alloy made by the conventional ingot casting process (i.e., the ingot cast alloy), the rapidly cooled alloy has a fine structure and has smaller crystal grain sizes. In addition, the crystal grains are distributed finely, the grain boundary has a wide area, and the R-rich phases are distributed thinly over the grain boundary. Such a good distribution of the R-rich phases improves the sinterability. That is why a rapidly cooled alloy has been used more and more often as a material to make an R-T-Q based rare-earth sintered magnet with good properties.
If a rare-earth alloy (especially a rapidly cooled alloy) is coarsely pulverized by a so-called “hydrogen pulverization process”, in which the alloy is made to occlude hydrogen gas once (and which will be referred to herein as a “hydrogen decrepitation process”), the R-rich phases present on the grain boundary will react with hydrogen and expand. As a result, the alloy tends to crack from the R-rich phase portions (i.e., grain boundary portions). Therefore, the R-rich phases tend to be exposed on the surfaces of powder particles, which have been obtained by pulverizing the rare-earth alloy by the hydrogen pulverization process. Besides, in the rapidly cooled alloy, the R-rich phases have such small sizes and have been distributed so uniformly that the R-rich phases are exposed on the surface of the hydrogen-pulverized powder particularly easily.
Such a pulverization method using the hydrogen decrepitation process is disclosed in U.S. patent application Ser. No. 09/503,738, for example.
A technique of substituting Dy, Tb, and/or Ho for a portion of a rare-earth element R to increase the coercivity of such an R-T-Q based rare-earth magnet is known. At least one element selected from the group consisting of Dy, Tb and Ho will be referred to herein as “RH”.
However, the element RH that has been added to the R-T-Q based rare-earth magnet material alloy will be present not only in the R2T14Q phase as the main phase but also in the grain boundary phase substantially uniformly after the molten alloy has been rapidly cooled. The element RH, present in those grain boundary phases, does not contribute to increasing the coercivity, which is a problem.
The high concentration of the element RH in the grain boundary will decrease the sinterability, which is also a problem. This problem becomes non-negligible if the ratio of the element RH to the overall material alloy is 1.5 at % or more and gets serious once this ratio has exceeded 2.0 at %.
Also, the grain boundary phase portions of the solidified alloy easily turn into a superfine powder (with particle sizes of 1 μm or less) as a result of a hydrogen decrepitation process and a fine pulverization process. Even if those portions have not changed into the superfine powder, they tend to have exposed powder surfaces. The superfine powder is likely to cause oxidation and firing problems and does affect the sinterability. That is why the superfine powder is usually removed during the pulverization process. A rare-earth element that is exposed on the surface of powder particles with particle sizes of 1 μm or more is oxidized easily and the element RH is oxidized more easily than Nd or Pr. Thus, the element RH, present in the grain boundary phase of the alloy, produces a chemically stable oxide and tends to get precipitated continuously in the grain boundary phase without substituting for the rare-earth element R in the main phase.
Consequently, portions of the element RH that are present in the grain boundary phase of a rapidly cooled alloy cannot be used effectively to increase the coercivity. The element RH is a rare-to-find element and is expensive, too. For that reason, to use valuable natural resources more efficiently and to cut down the manufacturing cost, it is strongly recommended to avoid such a waste of that precious element.
To overcome these problems, Patent Document No. 1 proposes that a rapidly cooled and solidified alloy, made by a strip casting process, be subjected to a heat treatment process at a temperature of 400° C. to 800° C. for 5 minutes to 12 hours to move the heavy rare-earth element from the grain boundary into the main phase and set the concentration of that element higher in the main phase.
Patent Documents Nos. 2 and 3 also disclose that the process of rapidly cooling a molten alloy should be controlled to regulate the structure of the resultant rapidly cooled alloy, not to increase the concentration of Dy in the main phase.
Specifically, Patent Document No. 2 proposes that in order to further reduce the grain size of the rapidly cooled alloy structure, the process of rapidly cooling a molten alloy be divided into the two stages of first cooling and second cooling and that the cooling rates in the respective stages be controlled within particular ranges.
Patent Document No. 3 proposes that just after having been made by getting a molten alloy cooled rapidly by a chill roller, a thin-strip rapidly cooled and solidified alloy be stored in a container to have its temperature controlled. According to the method disclosed in Patent Document No. 3, the average cooling rate is controlled to the range of 10° C./min to 300° C./min when the temperature of the alloy falls from 900° C. to 600° C. during the rapid cooling process, thereby controlling the distribution of the R-rich phases.                Patent Document No. 1: Japanese Patent Application No. 2003-507836,        Patent Document No. 2: Japanese Patent Application Laid-Open Publication No. 8-269643, and        Patent Document No. 3: Japanese Patent Application Laid-Open Publication No. 2002-266006.        